Metal casting process

ABSTRACT

AN IMPROVED REFRACTORY COMPOSITION FOR COATING INGOT MOLDS OF THE TYPE COMPRISING A REFRACTORY POWDER SLURRIED INTO AN AQUEOUS SILICA-CONTAINING BINDER IS AFFORDED BY INCORPORATION WITH SUCH REFRACTORY COMPOSITION SMALL AMOUNTS OF CHROMIUM AND/OR VANADIUM AND/OR MANGANESE CONTAINING SALTS.

United States Patent AU 112 EX U.S. Cl. 117-53 3 Claims ABSTRACT OF THE DISCLOSURE An improved refrwoay-somposition for com molds of the type comprising a rcfwder slurried into an a u inder is afforded by incorporation with such refractory composition small amounts of chromium and/or vanadium and/or manganese containing salts.

INTRODUCTION The instant invention relates to an improved metal casting process. More specifically, the present invention is concerned with a method of increasing the life of mold bottoms used to cast ingots, and prevent adherence of these mold bases to subsequently formed solidified ingots which must be removed from within the mold walls.

All metal ingots are cast from molds. One popular type is a big end down" mold. These molds rest on bases commonly known as stools. The stools are merely large, normally rectangular, flat slabs of metal, commonly made of cast iron, which are used as support for the mold sides and also, of course, form the bottom portion of the mold. The mold sides generally taper up in diameter from bottom to top. Another type of mold is known as the big end up mold. These are ladlelike receivers for the molten metal, the bottom portion of which molds are integral, nonremovable parts of the entire mold.

Various problems commonly occur in use of these molds and particularly with respect to the surface of their base portions. First, the unprotected metal surface quickly erodes and pits in the presence of molten metals which are cascaded upon their surface. Large gouges in the base portions are produced due to the force and high temperature developed by the flowing molten metal which contacts the surface of the stool. Since many molds are generally approximately 5-10 feet in height, the metal must be poured from a height at least equal. to that distance and quite often is poured from even greater heights. A considerable pressure head is thereby developed. Thus, the hot molten metal easily gouges gaping depressions in the base members under such force and at a temperature of at least the liquefaction temperature of the molten metal. Moreover, the problem of creation of pits or gouges in the base portions of the molds, caused by the above factors is aggravated due to the fact that the molten metal, especially near the bottom of the mold, remains in its erosive hot liquid state for a considerable amount of time subsequent to pouring.

The molten metal upon solidification to an ingot thereby has a bottom form conforming to the undesirable eroded surface configuration of the stool or base member of the mold. Thus, a considerable amount of the ingot, when withdrawn from the mold and subsequently processed into slabs or blooms, is lost through a cropping of the irregularly formed end of the slab. This, of course. is highly undesirable, since it results in 3,666,531 Patented May 30, 1972 undue loss of usable metal and increase in scrap, which must be subsequently reprocessed.

Another extremely serious and costly problem results after the ingot in the mold has solidified to a point where it can be removed from both the mold sides and its base platform member or stool. In many cases, if the surface of the stool is unprotected, or inadequately protected, and erosion occurs as described above, the ingot has a greater tendency to remain tightly adherent to the stool. Thus, after the mold sides are removed from around the ingot, which process can normally be efiiciently achieved with a minimal film of coating selected from a variety of coating agents, the ingot must be forcibly removed from the stool. This is normally achieved by raising both ingot and adherent stool, and thrusting them against some other larger object where= by the ingot is jarred loose. In many cases the stool and ingot are merely dropped on the floor from some suitable height. In such a situation, the stool is often broken into two or more smaller pieces and cannot be subsequently reused in casting other ingots. Again, replacement cost of these stools is high, making this aspect of the overall casting process somewhat disadvantageous. The same problem exists with respect to big end up molds wherein sticking of ingots particularly occurs at their base portion. New molds of this type are especially vulnerable to sticking due to their smooth surface unprotected by any layers of metal oxides or scale. A tight metal-to-metal bond between mold bottoms and ingots then occurs.

Cracking of molds and particularly their base portions due to the above discussed rough handling occasioned by stickers" between the base portions and ingots, is also enhanced by thermal shock during ingot formation. Unprotected or inadequately protected bottom surfaces of mold are especially susceptible to such destructive shock.

The above described problems have been solved to a large extent by the introduction of refractory composi tions which comprise a refractory powder slurried in an aqueous silica-containing binder and applying these compositions to at least the base memebr of an ingot mold and then allowing these compositions to dry. The thus treated mold is rendered immune to the erosion problem occasioned by the pouring of metal into such molds. The use of these refractory compositions are disclosed in the following U.S. patents, the disclosures of which are incorporated herein by reference: U.S. 3,184,815, U.S. 3, 509,936 and U.S. 3,184,813. A

While the compositions and methods disclosed in these patents have materially improved the erosion and sticking problems previously discussed, their use by the steel industry, for example, has indicated that certain characteristics of these formulas are ri'ot entirely satisfactory from the applicational standpoint.

For instance, it has been observed that when the refractory compositions are applied to molds whose temperature is in excess of 400 F. the coatings do not form a completely adherent bond to the mold surface. This means that the coatings, after metal is poured thereupon, tends to be loosened from the base or stool; thereby requiring complete retreatment when further pourings are to be made. In certain cases it has been found that coatings of the types described in the above listed patents tend to be removed from stools by contact with moisture such as heavy rains requiring application of additional coatings. This necessitates the use of more coating material to maintain a uniform film of the ceramic material on the base and other portions of the mold. If it were possible to improve the deficiencies discussed above when the refractory coatings described are used, it would be possible to provide better and more efficient ingot mold coatings.

3 OBJECTS OF T HE INVENTION The invention, therefore, has as an object the provision of the improved refractory coatings of the type described which may be applied to mold surfaces over a broad temperature range with a good ceramic bond being afforded.

Another object of the invention is to provide improved ceramic coatings of the type described which are adherent and are not subject to replacement due to contact with water.

Other objects will appear hereinafter.

THE INVENTION In accordance with the invention it has been found that the erosion of ingot molds may be prevented by coating such molds with a refractory composition which comprises a refractory powder slurried in an aqueous silica-containing binder which refractory composition contains a chromium and/or vanadium and/or manganese salt in an amount to provide from 0.005 to 5% by weight expressed as CrO V or Mn O based on the SIG, content of the silica-containing binder. The chromium, vanadium or manganese salts in a preferred embodiment of the invention are employed in an amount ranging from 0.05 to 5% by weight and, most preferably, from 0.1 to 3% by weight.

THE REFRACTORY POWDER The preferred refractories should include at least one refracto ry of ft ie fq l lgwl r g ty es: vitreous silica crystallinesilica, magnesi m silicate, a umtnt gi s1 icate alumina, graphite, zircom ro tum oxides dirtiest? and c ay riiayalso 'be' usdl'l'he's' fnaterialfs afe all well-known spb; stances and are all eaifinereian f available. Typical aluminum sa s gsiem lstmaxinsl tdsz i talaminated W of l fi iml il t a d msf ites .....r.th hs m s alumifium s t avail r m e .Island of .Mu 9r. artificially made by heating andalusite, silligfqapitg or cyanttg. Excellent magnesium silicates are forsterite or talcfwliile a useful zirconium silicate is zircon. The most preferred refractories, discussed in more detail hereinafter, are vitreous and crystalline silicas.

As mentioned above, two refractories have shown extreme promise for coating. Both of these materials are well-known and need little further elaboration. The first is a crystalline silica refractory. Representative types include sand, crystalline silica flour, crystalline silica grains, etc.

Other preferred refractory materials are those generally referred to as vitreous silicas. These are glassy modifications of silica, obtained by the fusion of selected lowtemperature crystalline forms, and are frequently referred to as quartz glass or silica glass. Specific vitreous silicas include those particles made from fused quartz glasses, silicate glasses, silica glasses such as the well-known Vycor" materials and fused silica glasses. With respect to all of these materials their thermal coefficients of expansion are relatively small in proportion to expansion properties of other refractories such as those of the sodalime and lead glass types. Generally, the above preferred refractories have thermal expansion coefficients smaller than 5 l0- cm./cm./ C. Also, the silica content of these granular siliceous refractory materials is generally greater than 96% silica expressed as SiO, and may range as high as 99.8% SiO- Thus, by the term vitreous silica is meant a refractory comprising a silica glass having a thermal coefiicient of expansion and SiO; content within the above ranges.

It has been determined that for best results in coating stools the refractory used in the silica slurry should be able to withstand severe heat shocks. Due to the extreme hot temperature of the molten metal as compared to that of the stool even when the latter is heated, an exceedingly abrupt change in temperature occurs when the metal contacts the stool. The coating must itself be able to withstand this heat shock to impart necessary protection to the stool base. It has been theorized that failure of some prior art materials was due, at least in part, to their inability to withstand this sudden increase in heat, thereby resulting in cracking of the coating and subsequent exposure of the metal surface to the cascading molten metal poured into the mold.

In view of the above it is generally thought that the most preferred silica-type refractories are those which have the highest silica content concomitant with the lowest thermal coefiicient of expansion. These properties are particularly possessed by vitreous silicas and more particularly those of the fused silica types. The latter materials have a silica content greater than 97% silica expressed as Si0 and a thermal coefiicient of expansion not greater than about 6X10 cm./cm./ C.

A typical fused silica of the type described above which is extremely useful in the practice of the invention, having a thermal coefficient of expansion of about 5x10 cm./cm./ C., has the following typical analysis:

Ingredients: Percent by weight sio 97.3 A1 0 1.7 Sub-oxides of silica 1.0

THE AQUEOUS SILICA-CONTAINING BINDER (A) The alkali-metal silicates The alkali metal silicates are a preferred classnLbindft ttse einsetsiaslys ss diuntstlicatemnmas t .lL=. 9-... nat e. preferenti l/minus. g fay ailability and low co 1s sodiu As set out above, the preferred source lie of silicate material is a sodium silicate. Commercial sources of this mate rial range from about 25% to about 40% by weight of solids of sodium silicate, generally supplied in aqueous form. More typically, they contain 30-40% by weight of solids. These commercially available silicates may be concentrated somewhat or diluted to any solids content as desired. Preferred alkali metal silicate binders for use in the invention are those materials containing 545% by weight of alkali metal silicate as a water solution.

While water alone may be used as the solubilizing agent for alkali metal silicate to form the liquid binders of the invention, it is understood that a binder may contain other solvents, polar in nature. It is only necessary that the organic solvents be compatible in water and do not deleteriously affect the alkali metal silicate, such by initiating polymerization of it to a solid, unusable, glassy mass.

Particularly preferred organic substances used in combination with water as solvents for the alkali metal silicate material are those which lower the freezing point of pure aqueous alkali metal silicates by their mixture with aqueous silicate solutions. Such products are especially useful. during the colder months of the year when they must be stored and/or used at relatively low temperature. Amines such as morpholine, di-ethyl amine, etc., and polyhydroxy organics as ethylene glycol, glycerine, etc., are preferred materials in making up solutions of alkali metal silicate binder.

A preferred binder, winterized against freezing contains 5-45% by weight of alkali metal silicate, 10-95% of water and 550% by weight of a polyhydroxy water compatible polar organic compound.

(B) Colloidal silica sols Another preferred binder used to form the slurries of the invention is a colloidal silica sol. These are well-known materials and are commercially available from several ticles which are dense, amorphous, and have an average particle diameter which does not exceed 150 millimicrons. As evidenced by a reading of Table I, all the silica sols contemplated as starting materials have an average par- Sources of Supply A typical group of commercially avai1a 5 ticle size diametenwell below 150 millimicrons Prefer-= ble silica sols that may be used in the practices of the ably, the Slarhhg Slhca sols have an average Parhcle slZo invention are those silica sols sold under the name -Naldiameter of from -50 millimicrohs- The Silica cohceh coag. Silica sols of this type are described below in tfalioh in the Sols y be between 01% and 60% by T bl I weight silica expressed as SiO More preferred sols con- TABLE I Silica sol I II III Iv v V] Percent colloidal silica as SiOz- -36 21-22 49- 50 35 pH 8.6 10.2 8.6 3.7 an 3.5 Viscosity at 77 F. cps 5 5 5 lU 20-30 6. 5 Specific gravity at 68 F 1. (m 1.205 1.255 1. 0b 1.385 1.255 Average surface area M2 per gram Ofsiol 330-430 190-210 135-190 ies-1st) 120-150 135-100 Average particle size, millimicrons 7.9 11-16 16-22 16-22 20-25 16-22 Density, lbs/gallon at 68 F-.. 9.1 10.0 10. 5 8. 8 ll. 6 10.5 N320, percent 0. 04 0. 0i 0. 10 O. 05 0.30 0. 01

Other silica sols that may be used in addition to those tain from 3.0 to 60% by weight of silica and most preferabove, may be prepared by using several well-known conably 10-60% by weight. ventional techniques. In a preferred practice of the inven- Other sols which may be employed as binders for the tion the silica sols are treated with a suitable base either silica refractory are those known as salt-free" silica sols. at the time of manufacture, or just prior to use, to provide These are particularly preferred when the suspension a pH of at least 11.0 and most preferably at least 11.5. media of the silica particles in the binder itself is solely These alkaline sols tend to promote adherence of the 30 a polar organic liquid or a mixture of water and polar coating to the stools. Perhaps, the most convenient method organic liquid. Since many of the above described sols of making aqueous colloidal silica sols is described in usually contain alkalimetal compounds as stabilizers, they Bird, US Pat. 2,244,355, wherein a dilute solution of are generally not compatible with organic systems due an alkali metal silicate is passed in contact with a cation to the fact that the alt present in th aqueous ol u e exchange resin in hydrogen form, whereby the silicate is gelation or precipitation of the silica particles when the converted to a dilute aqueous colloidal silica sol. The 0 aqueous phases are exchanged for polar organic solvents. dilute sol may be concentrated to solids concentrations Thi can be avoided by use of salt-free aqueous silica which are more economi lly Sable from the standpoint sols as starting materials in preparation of pure organo of shipping costs and ultimate process use, by emp y g sols or mixtures of water and organic as silica carriers. In the techniques described in either Bechtold et al. US .Pat. 40 order t id hi l i ff ct i i necessary h h g et US 2,630,721; of Alexander causative cations be removed from the surface of the 2,601,235- Another yP of Silica Sol which colloidally dispersed silica particles and from the liquid may be used in the practices of the invention is described phase f th ol, This may be readily accomplished by in e p ific i of Renter, Pot. 2,356,301 while treating typical silica sols of the type described in Becht aoueous colloidal silica may be used, it will be understood 1 et 1 S p 2 574 902 i a cation exchange that other forms of colloidal silica y be p y o Such in in the hydrogen form and a strong base anion exchange as for instance, $018 which contain a major Portlon of resin in hydroxide form. This treatment tends to produce Polar orgahlc solvohts- Said 5015 ey be generally a finished aqueous sol in which both the continuous aqueferred o as ol'gano Sole, and are yp y the sols ous phase of the sol and the particles of silica are con-- scribed 1h Marshall 2386,247- It only hoces sidered salt-free. Typical commercially available silica that the sllloa Partlcles used can be dlsporsed sols which may be deionized to produce salt-free silica loldally in a hydrophilic Substance, Such as Water or lower sols are those which are described in Table I above These yl alcohols h other organic compounds possesshlg aqueous salt-free silica sols may be either used as such Tolahvely fl dloleoh' cohstahtsin combination with one or more of the named refractor- In some instances mixtures of water and organic subies to constitute a slurry coating material or may be modistances compatible with water may be employed as susfied whereby the aqueous phase is completely or partially pending media for the colloidal silica particles. Particuexchanged for a hydrophilic polar liquid such as an alco larly preferred organic substances are those which lower hol or the alcohol is mixed with aqueous sol in desired the freezing point of pure aqueous sols by their admixproportions. The salt-free pure alcohols or aqueousture with these aqueous silica sols. Such final product alcoholic silica sols may then be easily combined with a sols then are especially useful during the colder nths refractory and the resultant slurry used to coat the stools. of the year when they must be stored and/()1- used at When the particle sizes Of the silica 5018 described above relatively low temperature. Amines such as morpholine, are Within the ranges Specified, the silica Particles p diethyl amine,etc., and polyhydroxy organics as ethylene in the starting aqueous or organic sol have specific surface glycol, glycerine etc., are preferred materials in making up areas of at least 20 and Usually in e S of 00 silica sols containing these substances as sole silica sus- 'f- Further; When deionized So18 are p y as a pending media or as a portion of a mixture additionally binder, y generally have a Salt Content expressed as containing water. A preferred sol, winterized against z n of less than 001%- freezing contains 5-50 parts by weight of polyhydroxy compound such as ethylene glycol, 20-85 parts by weight APPLICATION OF THE REFRACTORY of water and 10-60 parts by weight of silica. COMPOSITIONS Regardless of the method employed to produce the The slurry is provided in an amount adequate to form colloidal silica sol containing water, polar organic liquids a coating of sufiicient thickness to prevent the above menor mixtures of these substances as a continuous suspendtioned adherence and erosion from occurring. After the ing phase. it is desirable that said sols contain silica parslurry is allowed to dry with or without application of heat, whereby the liquid phase is driven from the surface of the base member, leaving a thin film of solid refractory coating, the molten metal is thereafter poured into the mold and on top of the now coated base member. The liquid metal is allowed to solidify into an ingot and then removed from the coated base member and mold sides.

The slurry coating reagent is simply applied to the stool or bottom of the mold by a wide variety of methods. For example, the slurry may be applied by flowing it over the stools, by spray techniques, by coating the stool with some type of applicator, etc. Spray application is believed to be the most efficient and practical way of slurry application. The slurry is best applied to the stool portion of big end down molds before the metal mold sides are placed thereon.

Effecting removal of the liquid phase of the slurry from the solid refractory material may likewise be carried out in a variety of methods. For example, the bottom portion of the mold may be coated with the slurry and allowed to dry gradually. Another method of laying down a thin protective coating is to apply the slurry to an already heated stool or mold having an integral base portion. This is particularly preferred in that the stools and their metal mold sides or big end up molds are generally already hot before introduction of the molten metal due to the residual heat from the previous casting run, and in such a method drying time is a very minimum period.

Another way of applying the protective film is to coat the base member, and then heat it as slowly or rapidly as desired to drive off the liquid portion of the slurry. In any event all that is necessary is that the coating be laid down in some manner after contact of the base member with the slurry.

In the most preferred method the coating slurry is applied to stools or mold base members having a temperature ranging from that of room temperature to l,000 F., and more preferably from 200 F. to 800 F. Best adherence of solid coating to stool is achieved by slurry application to the stools at a temperature range of ZOO-500 F. For best results, it has been determined that films should measure in thickness from 0.01" to 3" and most preferably from 0.01" to Ma.

It is believed that the excellent coating success achieved by use of the above described slurries is their ability to form a strong ceramic coating even when afiixed to the base members of the mold at relatively low temperatures, at least under foundry conditions, of say about 500 F. or even lower. The coating becomes completely resistant to subsequent contact with water and stays tightly adherent to the base portions of the mold even under such washings.

After the coating has been formed preparation for casting is complete except when big end down molds are used. In those cases the mold sides should be placed on the base member following coatings, if not already present during coating.

After coating operations, molten metal is poured into the mold, allowed to solidify into an ingot and the ingot is then separated from mold surfaces. The invention is not limited to use with specific mold sides or any particu lar metal mold bottom or to use with any particular molten metal. However, it has found specially preferred use in coating cast iron metal base members for molds which are used in forming ingots of steel.

RATIO OF REFRACTORY POWDER TO AQUEOUS SILICA-CONTAINING BINDER Regardless of the particular aqueous silica-containing binder used, the ratio of refractory powder to binder in making up the slurry may range from 2:1 to 1:50, the most preferred slurries containing refractory powder and more preferably crystalline or vitreous silica suspended in one or more of the above described binders in a ratio from 2:1 to 1:25.

8 THE CHROMIUM, VANADlUM AND MANGANESE SALTS The salts used in a referred practice of the invention H I EEQQEYF IP was. 22. 9 P ble directly dissolve the chromium, vanadium or manganese s silicacohtaining hinder, the inven; :553 at s mtx s the 'JWhi2 may. in. th drycrystalssthem ,ctorw :de.

COMPOSITIONS OF THE INVENTION To illustrate the typical compositions of the invention, Composition I is set forth below:

Composition I Ingredients: Percent by weight Water gallons 5 Sodium silicate solution containing 29% SiO do 1 Sodium chromate .4H O grams Fused silica lbs 52 The above composition was prepared by diluting the sodium silicate with the water and then adding the sodium chromate. Just prior to use, the fused silica was added and formed into a refractory slurry which had the consistency of house paint.

EXAMPLE I Composition I described above was applied to small cast iron blocks which were 2 /2" thick, 6" long and 2" Wide. One series of blocks was heated to 200 F., a second series to 400 F., and a third series to 700 F. In each case the coating was applied at these temperatures and allowed to cool at room temperature. They were then held under a water tap overnight. None of the slurry was removed.

Using the same test procedure, Composition I was prepared with the exception that the sodium chromate was omitted. The coatings applied to the 700 F. blocks were loosely adherent after cooling and partially removed by the stream of water.

EXAMPLE II To further illustrate the advantages of the compositions of the invention, Composition I was used to coat 300 stools which were used to cast ingot molds in a steel mill. The thus coated stools received poured steel and none of the stools were shown to have any stickers" occurring. The coating rate used in this particular coating test was 0.25#/ ingot ton.

At the same mill a chromate-free Composition I was applied to 100 ingots at the same dosage level with the result that a sticker rate of 25% was evidenced.

In testing the compositions of the invention under field conditions it was found that steel could be teemed onto the coated molds at higher temperatures without destruction or displacement of the coatings occurring. This pro vides an additional advantage or object of the invention.

Having thus described my invention it is claimed as follows:

1. In a process of the type wherein the erosion of a metal mold used to cast ingots from molten metal is inhibited by coating at least the base member of said molds with a refractory composition which comprises a refractory powder slurried in an aqueous silica-containing binder, the imprpovement which comprises adding to said refractory composition a metal salt from the group consisting of chromium, vanadium and manganese salts in an amount sufficient to provide from 0.005 to 5% by weight, expressed as CrO V or M o, based on the SiO: content of the silica-containing binder.

2. In process of claim 1 wherein the refractory powder is from the group consisting of vitreous silica, crystalline silica, magnesium silicate, aluminum silicate, alumina, graphite, zirconium silicate and clay, the aqueous silicacontaining binder is an alkali metal silicate and the chromium and vanadium salt is selected from the group consisting of alkali metal chromates and vanadates.

3. In a process of the type wherein the erosion of a metal mold used to case ingots from molten metal is inhibited by coating at least the base member of said molds with a refractory composition which comprises vitreous silica powder slurried into an aqueous sodium silicate solu tion, the improvement which comprises adding to said re- 10 amount sufficient to provide from 0.05 to 5% by weight, expressed as CrO based on the SiO content of the sodium silicate solution, with the ratio of vitreous silica to alkali metal silica-containing binder being in the range of from 2:1 to 1:50.

References Cited UNITED STATES PATENTS fractory composition a water-soluble chromate salt in an 106-3822, 38.27, 74, 84 

